1. Field of the Invention
The present invention relates to a method of optimizing a measurement condition in an NMR measurement.
2. Description of Related Art
FIGS. 1(a) and 1(b) illustrate the prior art method of optimizing an NMR measurement condition. FIG. 1(a) is a flowchart illustrating a general procedure for finding an optimum value of a measurement condition. FIG. 1(b) is a flowchart illustrating a procedure for finding optimum values of RF pulse widths. FIG. 1(c) is a diagram illustrating a pulse sequence for measurement of an RF pulse width.
The general procedure for finding an optimum value of a measurement condition is described by referring to FIG. 1(a). In step 1, NMR measurements are carried out while varying the value of a measurement condition to be optimized in given increments. In step 2, a graph for finding the optimum value from the obtained measurement data by appropriate processing is created. At this time, the varying value of the measurement condition is plotted on one axis of the graph. In step 3, the optimum value is found from the shape of the graph by visual estimation.
As a specific example, a procedure for finding an optimum value of an RF pulse width as a measurement condition is next described by referring to FIGS. 1(b) and 1(c). In step 1, NMR measurements are performed while varying the pulse width from 0 to 70 is using a pulse sequence shown in FIG. 1(c), for example, under measurement conditions listed in Table 1.
TABLE 1Measurement condition set 1 under which measurementsare performed with varying RF pulse widthItemValueSample15 mM copper dichloride/1% H2O,99% D2OMagnetic field intensity14.09636928 TObserved nucleus1HObserve frequency600.1723046 MHzCenter frequency of observationabout 4.7 ppm(resonance frequency of water)Number of data points16384Sweep width9.00252071 kHzNumber of accumulations1B1 pulse intensityabout 25 kHzObservation time1.81993472 sRelaxation_delay1 sTemperature25° C.
In the pulse sequence of FIG. 1(c), “[relaxation_delay]” indicates the wait time of each repetition pulse. In this example, the time is 1 s.
“[x_pulse]” indicates an RF pulse. In this example, NMR measurements are performed using a pulse width varied from 0 to 70 μs.
“[acquisition]” indicates an observation. In this example, the time necessary for an observation is 1.81993472 s as shown in Table 1.
Data obtained from a measurement is shown in FIG. 2, where one-dimensional (1D) NMR data obtained using a certain pulse width are arrayed in the order of values of pulse widths at intervals of 2 μs.
In step 2, the obtained NMR data are first Fourier-transformed. The resulting data are shown in FIG. 3, where the data are arrayed in the order of values of pulse widths at intervals of 2 μs in the same way as in FIG. 2. Then, with respect to each set of 1D NMR data, a range from 4 to 5.5 ppm in the signal region is displayed. The data are arrayed horizontally in the order of values of pulse widths. The obtained graph is shown in FIG. 4, where the horizontal axis indicates the pulse width, while the vertical axis indicates the intensity of the NMR spectrum.
In step 3, a waveform formed by connecting the vertices of spectral intensities of FIG. 4 by means of straight lines is regarded as a sinusoidal (SIN) wave. Visual estimation of a pulse width of 360° reveals that it is about 28 μs. Since it has been already known that the optimum pulse width is 90°, the optimum pulse width is equal to the pulse width of 360° divided by 4, i.e., 28 μs/4=7 μs.
An NMR instrument designed to quantitatively indicate the nonuniformities in transmit and receive magnetic fields is shown in Japanese Patent Laid-Open No. H3-139330. In particular, NMR scans are made with RF exciting field intensities of different arrays. A curve is applied to each set of corresponding data elements in one set of intensity arrays. The peaks of the applied curves are determined. Corresponding data in the transmit and receive arrays are generated from the determined peaks. Thus, a magnetic field map indicating nonuniformities in an RF magnetic field by means of the magnitudes of data elements is created.
However, the prior art method of finding the optimum RF pulse width has the problem that the reliability of the optimum value is low because the value is found from a created graph by visual estimation. In order to obtain an optimum value with high reliability, it is necessary to increase the number of measurement data items. This prolongs the measurement time. If the number of measurement data items is reduced to shorten the measurement time, the reliability of the obtained optimum value deteriorates.
Furthermore, Japanese Patent Laid-Open No. H3-139330 does not disclose a technique for optimizing measurement conditions, though the reference discloses a technique quantitatively indicating nonuniformities in transmit and receive fields.